This invention relates to superconducting windings in general and more particularly to a switching device for shorting at least one superconducting magnet winding.
A switching device for shorting at least one superconducting magnet with at least two contacts which can be cooled by a cryogenic medium and can be joined together by means of an actuator in such a way that a superconducting current path and a normally conducting path which completely stabilizes the superconducting current path are developed, is described in DE-OS No. 25 21 328. As soon as the magnetic field of a superconducting magnet winding, especially a high field intensity magnet coil, is once built up, no further electric energy need be supplied from the outside to maintain the field of the coil. Then, only the energy required for the refrigeration equipment needed for maintaining the superconductive state of the winding must still be supplied. For storing the electric energy fed into the magnet winding, the winding can therefore be short-circuited at its ends by means of a switching device, also called persistent switching device, with as low a resistance as possible. The current then flows almost unattenuated in the shorted circuit so formed, and the power supply required for exciting the magnet winding can thus be interrupted.
The switching device described in DE-OS No. 25 21 328 mentioned above contains two contacts, each of which comprises a support element of normally conducting material acting as a matrix in which mutually parallel superconducting parts are embedded. The contacts are kept indirectly by at the operating temperature of the superconducting conductor sections by a cryogenic coolant. A mechanical actuator required for opening and closing the switching device is designed so that the respective superconducting parts as well as their normally conducting parts can be joined together to thus form a superconducting current path and a normally conducting current path which completely stabilizes the superconducting current path. Complete stabilization is understood here to mean a sufficiently large conductor cross section of the normally conducting current path shunted across the superconducting current path so that the entire operating current can be carried, at least for a short time, without damage to the switching device, if the superconducting current path becomes normally conducting.
In this switching device, the surface of the one contact is made curved, while the other contact may have, for instance, a plane or concave surface. To assure a low contact resistance between the two contacts, a relatively high contact pressure is required, in addition to an exact guidance of the contacts. Even so, it cannot always be assured that all corresponding superconducting conductor sections of the two contacts are joined together with about the same contact pressure and thereby cause approximately the same contact resistances in the corresponding superconducting current path. It must rather be assumed that some superconducting conductor sections are joined together with only relatively small contact pressure and thus lead to correspondingly high contact resistances. With the known switching device, reproducible resistances in the order of 10.sup.-9 ohms or less cannot be assured definitely for extended periods of time. Such low resistances, however, are required especially for switching devices for shorting large superconducting magnets such as must be provided, for instance, for nuclear fusion equipment or for nuclear spin tomography (nuclear magnetic resonance tomography).
Besides such mechanical switching devices with the problem of long-term stability and reproducibility of the resistance in the closed state, superconducting switching devices also are known, the superconducting current path of which is put in normal conduction by a separate heating device for developing a quasi-open state of the switching device (see DE-OS No. 31 35 177). If highly stabilized superconductors are used for such switching devices, the problem of too low a resistance in the open switch position arises, so that only a correspondingly slow excitation of the superconducting magnet winding is possible. If, therefore, only partially stabilized superconductors are used, this means an accordingly low current carrying capacity in the closed state. While a higher resistance of the switching device in the open state and a generally sufficient current carrying capacity in the closed state are achieved, if a poorly conducting stabilizing material such as a copper-nickel alloy is used instead of copper, for fast excitation of, in particular, large magnets with high inductance, however, the resistance of the switching device in the open state is also not sufficient here. Still higher contact resistances between the copper-nickel stabilized switch material and the generally copper stabilized conductor of the magnet cause further difficulties to appear.
It is therefore an object of the present invention to improve a switching device of the type mentioned at the outset in such a direction that it will result in a short circuit of the superconducting magnet winding with a very low resistance of 10.sup.-9 ohms or less and a sufficiently high resistance of, for instance, 200 ohms in the open state in a relatively simple manner. Here, sufficient stabilization and sufficiently high current carrying capacity the switching device in the closed state are assured.